Trichostomum portoricense is a distinctive species with no immediate relatives in the genus. The deep, narrow groove down the adaxial surface of the leaf is reminiscent of that of species of Barbula or Anoectangium. The massive, caplike papillae are similar to those of Tuerckheimia, but the long-ligulate leaf shape of T. portoricense is diagnostic. The leaf lamina fragments easily in patches, and doubtless figures in asexual reproduction. The distribution was recently summarized by P. M. Eckel (2003).